Solid oxide electrochemical devices have demonstrated great potential for future power generation with high efficiency and low emission. Such solid oxide electrochemical devices include solid oxide fuel cells (SOFCs) for power generation and solid oxide electrolyzers for chemical (e.g. H2, O2, and CO) production.
In an SOFC, stacks of fuel cells, each of which is capable of generating a small amount of power, are connected together. Each cell is connected to its neighboring cell with an interconnect, which serves as both a current collector and a channel for flowing gases to the electrodes. Two basic cell constructions are used for SOFCs: electrolyte-supported cells and electrode-supported cells. In electrode-supported cells, the support electrode functions include electrical flow path, mass transport path, and mechanical support. To satisfy these functions, the support-electrode must have sufficient conductive components, porosity, and strength.
Typical support-electrodes must be considerably thick to provide the required mechanical strength and handling ability. In electrode-supported cell fabrication, the differences in sintering densification behavior and coefficient of thermal expansion (CTE) of the electrode and electrolyte components result in non-flatness (such as camber shape and edge ripples) of the cell. Generally, as support electrode thickness increases, the cell cambering tends to be reduced. The low mechanical strength and cell non-flatness in electrode-supported cells can lead to cell fracture in fabrication, stack assembly, and operation. While mechanical strength and cell flatness favor a thick support electrode, thick support-electrodes can restrict mass transport through the electrodes by limiting the oxygen transport in the support cathode or fuel/product transport in the support anode. The limitations in mass transport will lead to lower cell/stack performance, especially at high reactant utilizations for high efficiency. One approach to improve mass transport through the thick electrode is to increase porosity; however, the mechanical strength of the electrode will be compromised by too much porosity.
Previous attempts to address these problems have fallen short. For example, use of a composition gradient permits use of thicker and better performing anodes; however, the CTE mismatch between NiO and zirconia creates challenges in fabricating large, flat electrode-supported cells. The large volume of Ni in the anode after reduction could also result in creeping and sintering of the anode under the high operating temperatures of SOFCs.
An alternative structure is to form a continuous three-dimensional network with many microcomposite NiO and zirconia subelements in patterns. The network improves the electrical connectivity and increases the strength of the overall structure; however, effectively controlling the desired order of the subelements is difficult as they are vulnerable to distortion forces in the fabrication process.
Although these efforts show promise in improving the electrochemical and mechanical performance of electrode-supported cells, problems still remain. Accordingly, there is a need for an electrode-supported cell with improved electrochemical and mechanical performance.